I can't add a new category. When I type one and press enter or "add"...

I can't add a new category anymore. When I type one and press enter or "add" in the category widget box on "write" page, it disappears, and nothing happens. So I have to scroll down all the categories and check the right small boxes to appley categories which already exists, because until recently I can make as many ones as I wish in the type-in box, to a new past. I use IE7 and I have tried clearing cache and cookies, and adding another adiminstrator and loggin in, but nothing works. I am afraid it is that I have made too many categories. Is there a limit to the number of categories we can create? Am I deprived the feature of adding a new category?
There is another problem on this write page. When I fill in a title, and wrote a new post and press "save and continue " button, a new simple text page opens and it says," wordpress Are you sure you want to edit this post: ""? no yes ". Wheter I choose no, or yes, I am forced to go to a new write page and the new past I might have saved has all gone. I am brought back to a blank new write page.
I changed IE to Firefox, and crear cache and cookies, but this time the system says," You don't have no permission to do that" when I add a new category either on write page or edit category page, though I am the only adiminstrator.

The permission denied problems are usually because the browser cache did not flush. Performing a force reload and/or emptying the cache usually fixes it. If doing those things does not help, then please send a feedback to staff including all the details.

Reference:
This is the primary place for entering new categories.
Dashboard -> Manage -> Categories -> Add Category

And this is where you add new Categories "on the fly" while writing a post.

I am afraid it is that I have made too many categories. Is there a limit to the number of categories we can create?

When you run into the "are you sure you want to navigate away from this page" warning, the solution is to copy everything in the text box then, while it is still highlighted (selected) hit Publish. For some reason this forces it to go through fine. If something screws up, you've got the entire post on your clipboard.